Disposable absorbent products typically contain an absorbent batt of cellulosic fibers. The absorbency of such products have been improved by incorporating superabsorbent materials into the absorbent batt. Superabsorbent materials, also referred to as hydrogels, may be based on acrylate and methacrylate polymers and copolymers, starch or a modified starch, hydrolyzed cross-linked polyacrylamides, cross-linked sulfonated polystyrenes or maleic anhydride, and polyoxyalkylene glycols. For example, absorbent polyoxyalkylene glycols may be made from isocyanate-capped polyesters or poly-(oxyalkylene) glycols and difunctional extenders.
If utilized with an absorbent batt, superabsorbent material is usually added to the absorbent batt in particulate form. Unfortunately, particulate superabsorbents often change location in or fall out of the absorbent batt and/or exhibit gel-blocking, a phenomenon which inhibits migration of fluid to available superabsorbent particles. Consequently, thermoplastic superabsorbent polymers which can be melt processed into nonwoven fibrous webs are highly desirable.
Generally speaking, thermoplastic superabsorbent polymers are formed from water soluble polymer segments which are joined by chemical bonds that allow the water soluble polymer segments to swell and absorb water without dissolving. High molecular weights are usually necessary for such polymers to have superabsorbent properties as well as strength and toughness. The high molecular weight polymers typically have high viscosities and must be processed at high temperatures which improve the processability but may partially degrade the polymer. The resulting products may contain polymers that are partially degraded causing some loss of desired physical properties. In some situations, high molecular weight polymers may be partially degraded by overheating to reduce the viscosity of the polymer to a useful range for processing resulting in superabsorbent materials that have low gel strength and absorbent capacity, and high levels of extractables.
Aromatic urethane bonds are known to evanesce at elevated temperatures leaving behind aromatic alcohols and aromatic isocyanates. This phenomenon is used to form water-based urethane coatings that are stable at room temperature. Such coatings contain isocyanates which have been reacted with a material such as, for example, phenol, to create a reversible aromatic urethane bond. Isocyanates that have been blocked in this manner are unable to react at room temperature with compounds that normally react with isocyanates but will react at temperatures which cause the aromatic urethane bond to evanesce.